Transformer-Less UPFC for Wind Turbine Applications

45 International Journal for Modern Trends in Science and Technology
Volume: 2 | Issue: 06 | June 2016 | ISSN: 2455-3778IJMTST
Transformer-Less UPFC for Wind Turbine
Applications
Aparajita Pandey1
| Durga Sharma2
1PG Scholar (Power Systems), Dept. of Elec. Engg., Dr. CVRU, Bilaspur, Chhattisgarh, India
2HOD, Dept. of Elec. Engg., Dr. CVRU, Bilaspur, Chhattisgarh, India
In this paper, an innovative technique with a new concept of transformer-less unified power flow controller
(UPFC) is implemented. The construction of the conventional UPFC that consists of two back-to-back inverters
which results in complexity and bulkiness which involves the transformers which are complication for
isolation & attaining high power rating with required output waveforms. To reduce a above problem to a
certain extent, a innovative transformer-less UPFC based on less complex configuration with two cascade
multilevel inverters (CMIs) has been proposed. Unified power flow controller (UPFC) has been the most
versatile Flexible AC Transmission System (FACTS) device due to its ability to control real and reactive power
80w on transmission lines while controlling the voltage of the bus to which it is connected. UPFC being a
multi-variable power system controller it is necessary to analyze its effect on power system operation. The
new UPFC offers several merits over the traditional technology, such as Transformer-less, Light weight, High
efficiency, Low cost & Fast dynamic response. This paper mainly highlights the modulation and control for
this innovative transformer-less UPFC, involving desired fundamental frequency modulation (FFM) for low
total harmonic distortion (THD), independent active and reactive power control over the transmission line,
dc-link voltage balance control, etc. The unique capabilities of the UPFC in multiple line compensation are
integrated into a generalized power flow controller that is able to maintain prescribed, and independently
controllable, real power & reactive power flow in the line. UPFC simply controls the magnitude and angular
position of the injected voltage in real time so as to maintain or vary the real and reactive power flow in the
line to satisfy load demand & system operating conditions. UPFC can control various power system
parameters, such as bus voltages and line flows. The impact of UPFC control modes and settings on the
power system reliability has not been addressed sufficiently yet. Cascade multilevel inverters has been
proposed to have an overview of producing the light weight STATCOM’s which enhances the power quality at
the output levels.When the multilevel converter is applied to STATCOM, each of the cascaded H-bridge
converters should be equipped with a galvanically isolated and floating dc capacitor without any power
source or circuit. This enables to eliminate a bulky, heavy, and costly line-frequency transformer from the
cascade STATCOM. When no UPFC is installed, interruption of either three-phase line due to a fault reduces
an active power flow to half, because the line impedance becomes double before the interruption. Installing
the UPFC makes it possible to control an amount of active power flowing through the transmission system.
Results has been shown through MATLAB Simulink
KEYWORDS: FACTS, UPFC, CMIs, etc.
Copyright © 2016 International Journal for Modern Trends in Science and Technology
All rights reserved.
I. INTRODUCTION
Power systems in general are interconnected for
economic, security and reliability reasons.
Exchange of contracted amounts of real power has
been in vogue for a long time for economic and
security reasons. To control the power flow on tie
lines connecting controls areas, power flow control
equipment such as phase shifters are installed.
They direct real power between control areas. The
interchange of real power is usually done on an
hourly basis. On the other hand, reactive power
flow control on tie lines is also very important.
Reactive power flow control on transmission lines
ABSTRACT

Transformer-Less UPFC for Wind Turbine Applications
connecting different areas is necessary to regulate
remote end voltages. Though local control actions
within an area are the most effective during
contingencies, occasions may arise when adjacent
control areas may be called upon to provide
reactive power to avoid low voltages and improve
system security. This schedule should conform to
the provisions of the relevant interconnection
agreements and may provide for: (a) The minimum
and maximum voltage at stations at or near
terminals of inter-area tie lines (b) The receipt of
reactive power flow at one tie point in exchange for
delivery at another (c) The sharing of reactive
requirements of tie lines and series regulating
equipment (d) The transfer of reactive power from
one area to another. The above statements clearly
calls upon the power flow regulating equipment to
not only be able to control red power but also
simultaneously control reactive power flow rapidly.
Further, the voltage at stations at or near terminals
of inter-area tie lines should be controlled within
limits. Power flow in a network is not easily
controlled because line parameters that determine
the flow of power in the system are difficult to
control. Fortunately, the ability to control power
flow at the transmission level has greatly been
influenced by the advances made in the field of
high power switching devices. Solid state devices
provide transmission utilities the flexibility to
control the system power flows. Today, with the
availability of high power gate turn-off thyristors
(GTO) it has become possible to look beyond the
realm of conventional thyristors for power flow
control. These devices are broadly referred to as
Flexible AC Transmission Systems (FACTS).
Control of any of the above parameters cm help to
control the power flow and the process is known as
compensation. FACTS devices could be placed
either in series or in shunt with the transmission
line with the intention of controlling the power flow
in it. If the transmission line impedance is modified
by the addition of FACTS, it is termed as series
compensation. If the phase angle difference is
modified, it is termed as phase angle
compensation. Shunt compensation, in which the
FACTS device is placed in parallel, is mainly used
to improve the system voltage characteristics.
Static variable compensator (SVC) belongs to this
family of FACTS devices.
II. DETAILS ABOUT UPFC
To understand the unified power flow concept,
consider a power system with two machines
connected by a transmission line of reactance X,
(purely inductive) along with two voltage sources
Vsh and Vse representing the UPFC as shown in Fig.
1.3. The voltage sources denoted by Vsh and Vse in
the Fig. 1 are connected in shunt and series
respectively at the mid- point of the transmission
line. Voltage source Vsh, is connected to the
transmission line through a transformer
represented as a reactance Xsh. It is assumed that
the voltage sources denoted by Vsh and Vse have the
capabilities of varying their magnitude and their
phase angle.
Figure 1 A power system with two machines connected by
a transmission line with voltage source Vsh and Vse
representing the UPFC
To understand the operation of the source Vsh the
source Vse is disconnected Reactive power flows
from the voltage source Vsh to bus M if the
magnitude of the voltage source Vsh is greater than
the mid-point voltage VM, and the phase of them
are the same. If the phase angle of the voltage
source Vsh leads the phase angle of mid-point
voltage VM, and the magnitude of Vsh, is greater
than VM, then real and reactive power will flow from
the voltage source Vsh to the bus M. Conversely, if
the magnitude of the shunt voltage Vsh is less than
the mid-point voltage VM but the phase angle
difference between them is zero, then only reactive
power will flow from the bus M to the bus P. In this
process, the voltage source Vsh is consuming
reactive power. If the phase angle of VM leads the
phase angle of Vsh then both real and reactive
power will flow from bus M to bus P and the voltage
source is said to be consuming both real and
reactive power. In summary, by controlling the
magnitude and phase angle of the shunt voltage
source Vsh the direction of real and reactive power
flow to the bus M can be controlled. Alternatively,
the voltage source Vsh can be made to function as a
load or as a generator for the power system. In the
above operation, if the phase angle difference
between the voltage at bus M and that of Vsh is
maintained at zero, then by varying the magnitude

of Vsh reactive power cm either be consumed or
generated by Vsh. This operation cm be compared
with that of a thyristor controller reactor with fixed
capacitor (shunt compensator) that generates or
absorbs reactive power by altering its shunt
reactive impedance. It should be noticed that the
function of a shunt compensator is being
duplicated by the voltage source Vsh.
III. LITERATURE SURVEY
L. Gyugyi, T.R. Rietman, A. Edris, C.D.
Schauder & S.L. Williams in 1995 proposes a New
Approach to Power Transmission Control that
shows that the Unified Power Flow Controller
(UPFC) is able to control both the transmitted real
power and, independently, the reactive power flows
at the sending- and the receiving end of the
transmission line. The unique capabilities of the
UPFC in multiple lines Compensation are
integrated into a generalized power flow controller
that is able to maintain prescribed, and
independently controllable, real power and reactive
per flow in the line. The paper describes the basic
concepts of the proposed generalized P and Q
controller and compares it to the more
conventional, but related power flow controller
such as the Thyristor-Controlled Series Capacitor
and Thyristor-Controlled Phase Angle Regulator.
[2]
A. Rajabi-Ghahnavieh, M. Fotuhi-Firuzabad,
M. Shahidehpour & R. Feuillet in 2010
implements a UPFC for Enhancing Power System
Reliability that discusses various aspects of unified
power flow controller (UPFC) control modes and
settings and evaluates their impacts on the power
system reliability. UPFC is the most versatile
flexible ac transmission system device ever applied
to improve the power system operation and
delivery. It can control various power system
parameters, such as bus voltages and line flows.
The impact of UPFC control modes and settings on
the power system reliability has not been
addressed sufficiently yet. [3]
Hideaki Fujita, Yasuhiro Watanabe, &
Hirofumi Akagi in 1999 proposes Control and
Analysis of a Unified Power Flow Controller that
presents a control scheme and comprehensive
analysis for a unified power flow controller (UPFC)
on the basis of theory, computer simulation, and
experiment. This developed theoretical analysis
reveals that a conventional power-feedback control
scheme makes the UPFC induce power fluctuation
in transient states. The conventional control
scheme cannot attenuate the power fluctuation,
and so the time constant of damping is
independent of active- and reactive-power feedback
gains integrated in its control circuit. [4]
Mahmoud A. Sayed and Takaharu Takeshita in
2014 proposes Line Loss Minimization in Isolated
Substations and Multiple Loop Distribution
Systems Using the UPFC in which the line loss
minimum conditions in isolated substations and
same substation multiple loop distribution
systems by using the unified power flow controller
(UPFC). In each case, the mathematical model is
presented and the line loss minimum conditions
are obtained based on the line parameters of the
distribution feeders. Since multiple loop
distribution system is fed from same substation,
the line loss minimization can be achieved by
compensating the summation of the line reactance
voltage drop. In an isolated substation loop
distribution system, the line loss minimization can
be achieved by compensating the summation of the
line reactance voltage drop in addition to the
voltage difference of the substations. Realization of
both cases can be achieved if the loop current is
eliminated from the loop system. [5]
Hideaki Fujita, Yasuhiro Watanabe and
Hirofumi Akagi in 1999 implements Transient
Analysis of a Unified Power Flow Controller, and its
Application to Design of the DC-Link Capacitor in
which it shows a transient analysis of a unified
power flow controller (UPFC), and design of
capacitance of the dc-link capacitor based on that
analysis. Active power flowing out of the series
device in transient states is theoretically discussed
to derive what amount of electric energy the dc link
capacitor absorbs or releases through the series
device. As a result, it is clarified that the active
power flowing out of the series device is stored in
the line inductance as magnetic energy during
transient states. [6]
Hideaki Fujita, Hirofumi Akagi and Yasuhiro
Watanabe in 2006 shows Dynamic Control and
Performance of a Unified Power Flow Controller for
Stabilizing an AC Transmission System which
presents dynamic control and performance of a
unified power flow controller (UPFC) intended for
installation on a transmission system consisting of
two sets of three-phase transmission lines in
parallel. When no UPFC is installed, interruption of
either three-phase line due to a fault reduces an
active power flow to half, because the line
impedance becomes double before the
interruption. [7]
Liming Liu, Pengcheng Zhu, Yong Kang, and
Jian Chen in 2007 proposes Power-Flow Control

Performance Analysis of a Unified Power-Flow
Controller in a Novel Control Scheme shows the
real, reactive power, and voltage balance of the
unified power-flow control (UPFC) system is
analyzed. Two important results related to UPFC
control are shown in this paper. First, the shunt
converter provides all of the required reactive
power during the power-flow changes if the UPFC
bus voltage is constant. Second, the UPFC bus
voltage can be controlled both from the sending
side and from the receiving side. Based on the
analysis, a novel coordination controller is
proposed for the UPFC. The basic control strategy
is such that the shunt converter controls the
transmission-line reactive power flow and the
dc-link voltage. The series converter controls the
transmission-line real power flow and the UPFC
bus voltage. [8]
S. Kannan, Shesha Jayaram and M. M. A.
Salama in 2004 proposes a new real and reactive
power coordination controller for a unified power
flow controller (UPFC). The basic control for the
UPFC is such that the series converter of the UPFC
controls the transmission line real/reactive power
flow and the shunt converter of the UPFC controls
the UPFC bus voltage/shunt reactive power and
the DC link capacitor voltage. In steady state, the
real power demand of the series converter is
supplied by the shunt converter of the UPFC. To
avoid instability/loss of DC link capacitor voltage
during transient conditions, a new real power
coordination controller has been designed. The
need for reactive power coordination controller for
UPFC arises from the fact that excessive bus
voltage excursions occur during reactive power
transfers. [9]
IV. OBJECTIVE
The main objective of this paper is implementing
the Unified Power Flow Controller without utilizing
the transformer either at the source/grid side or at
the load side. The utilizing of the transformer may
results in complexity in the circuit resembling in
the isolation. Apart from the above, the UPFC uses
the dual CMI devices at both the ends supporting
the grid as well as the load resulting in less weight
& high efficiency. In this concept, the FFM method
is also used which segregates. The UPFC can
provide simultaneous control of all basic power
system parameters (transmission voltage,
impedance and phase angle). The controller can
fulfill functions of reactive shunt compensation,
series compensation and phase shifting meeting
multiple control objectives. From a functional
perspective, the objectives are met by applying a
boosting transformer injected voltage and a
exciting transformer reactive current. The injected
voltage is inserted by a series transformer. Besides
transformers, the general structure of UPFC
contains also a "back to back" AC to DC voltage
source converters operated from a common DC link
capacitor. First converter (CONV1) is connected in
shunt and the second one (CONV2) in series with
the line. The shunt converter is primarily used to
provide active power demand of the series
converter through a common DC link. Converter 1
can also generate or absorb reactive power, if it is
desired, and thereby provide independent shunt
reactive compensation for the line. Converter 2
provides the main function of the UPFC by injecting
a voltage with controllable magnitude and phase
angle in series with the line via an voltage source.
The reactance XS describes a reactance seen from
terminals of the series transformer.
V. PROBLEM IDENTIFICATION
In our concept of UPFC in which the main
challenges that arises while designing the simulink
of the concept are (a) Total Harmonic Distortion, (b)
Efficiency, (c) Undershoots & Overshoots and (d)
Settling Time
VI. METHODOLOGY
A) Methodology for Conventional UPFC:
In Conventional UPFC, the controller has
been designed with the help of transformer for
isolation purpose apart from which the operation of
the UPFC along with the transformer has efficient
output but it costs too high as well as the circuit
becomes too bulky and heavier as shown in Figure
2.
Figure 2 Conventional UPFC
In the Proposed model of the UPFC is following the
concept of transformer less which enhances the
performance of the model without any sort of
bulkiness and less complexity as well as with less
cost. In this two CMIs are used to provide proper

transmission between grids as well as the load.
Total harmonic distortion has been minimized to
certain extent as shown in Figure 3.
Figure 3 Proposed UPFC
With respect to the Figure 4, the CMIs are
connected between the grid and load. Here a circuit
breaker is been implemented to verify the actual
waveforms at the input and output with or without
the effect of UPFC.
Figure 4 Schematic Diagram of Proposed UPFC
The following simulation parameters has been
considered while designing the proposed model,
PARAMETERS VALUES
Grid Voltage 480V
Rated Frequency 60Hz
Transformer 1 480V / 4160V, 75KVA
Transformer 2 480V / 4160V, 75KVA
Rated Line Current 10A
X1 2.5mH
X2 3.2mH
Table 1 Simulation Parameters
VII. PROPOSED MODEL
This paper present a modulation and control
method for the transformer-less UPFC, which has
the following, features:
(a) Fundamental frequency modulation (FFM) of
the CMI has extremely low THD of output voltage,
low switching loss and high efficiency.
(b) All UPFC functions, such as voltage regulation,
line impedance compensation, phase shifting or
simultaneous control of voltage, impedance, and
phase angle, thus achieving independent active
and reactive power flow control over the
transmission line DC capacitor voltage balancing
control for both series and shunt CMIs.
(c) Fast dynamic response (<10 ms). The
transformer-less UPFC with proposed modulation
and control can be installed anywhere in the grid to
maximize/optimize energy transmission over the
existing grids, reduce transmission congestion and
enable high penetration of renewable energy
sources.
(d) We can replace the source with Wind Turbine
Generator also we can consider the battery source
between two inverters.
(e) Apart from the above, THD can be also
minimized to certain extent.
VIII. CONCLUSION, RESULTS & DISCUSSIONS
Figure 5 Input Waveform of conventional before using
UPFC
Figure 6 Output Waveform of conventional before using
UPFC

As the figure 5, the input waveform is observed and
the output waveform is obtained in figure 6 in
which the sag is being produced between the
interval 0.05 to 0.15 which is been minimized to
certain extent using our proposed model which is
shown in figure 7.
Figure 7 Output Waveform of conventional after using
UPFC
Figure 8 THD of the voltage at the load after using UPFC
Figure 9 THD of the current at the load after using UPFC
REFERENCES
[1] N. G. Hingorani and L. Gyugyi, UnderStanding FACTS:
concept and technology of flexible AC transmission
systems. New York: IEEE Press, 2000.
[2] L. Gyugyi, C. D. Schauder, S. L. Williams, T. R.
Rietman, D. R. Torgerson, and A. Edris, “The unified
power flow controller: A new approach to power
transmission control, ” IEEE Trans. Power Del., vol. 10,
no. 2, pp. 1085–1097, Apr. 1995.
[3] A. Rajabi - Ghahnavieh, M. Fotuhi - Firuzabad, M.
Shahidehpour, and R. Feuillet, "UPFC for enhancing
power system reliability," IEEE Trans. Power Del., vol.
25, no. 4, pp. 2881–2890, 2010.
[4] H. Fujita, Y. Watanabe and H. Akagi, “Control and
analysis of a unified power flow controller,” IEEE
Trans. Power Electron., vol. 14, pp. 1021–1027, 1999.
[5] M. A. Sayed, and T. Takeshita, “Line loss minimization
in isolated substations and multiple loop distribution
systems using the UPFC,” IEEE Trans. Power Electron.,
vol. 29, no. 11, pp. 5813–5822, Jul. 2014.
[6] H. Fujita, Y. Watanable, and H. Akagi, “Transient
analysis of a unified power flow controller and its
application to design of dc-link capacitor, ” IEEE Trans.
Power Electron., vol. 16, no. 5, pp. 735–740, Sept.
2001.
[7] H. Fujita, H. Akagi, and Y. Watanable, “Dynamic
control and performance of a unified power flow
controller for stabilizing an AC transmission system,”
IEEE Trans. Power Electron., vol. 21, no. 4, pp.
1013–1020, Jul. 2006.
[8] Liming Liu, Pengcheng Zhu, Yong Kang, and Jian
Chen, “Power-flow control performance analysis of a
unified power-flow controller in a novel control
scheme, ” IEEE Trans. Power Del., vol. 22, no. 3, pp.
1613–1619, Jul. 2007.
[9] S. Kanna, S. Jayaram, and M. M. A. Salama, “Real and
reactive power coordination for a unified power flow
controller, ” IEEE Trans. Power Syst., vol. 19, no. 3, pp.
1454–1461, Aug. 2004.
[10]J. Z. Bebic, P. W. Lehn, and M. R. Iravani, “P-δ
characteristics for the unified power flow controller–
analysis inclusive of equipment ratings and line limits,
” IEEE Trans. Power Del., vol. 18, no. 3, pp.
1066–1072, Jul. 2003.

Transformer-Less UPFC for Wind Turbine Applications

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